Pull boring has been known for many years and consists essentially of pulling a rotating cutter head axially through a tubular member to machine cut the inner surface of the member. The members being machined are usually of considerable length. It is also usually desirable that the machining operation be done speedily and give a desired fine surface finish. Preferably the operation should be a single cut operation.
Members which are machined in this way are borehole linings and cylinders for hydraulic piston and cylinder assemblies.
The cutter heads generally used today comprise a body adapted to be coupled to the driving shaft and carrying a series of cutter tips. The cutting edges of the tips extend radially and are parallel to the direction of travel in use. The cutting tips extend rearwardly from the cutting edges parallel to the axis of the cutter head.
There are a plurality of cutting tips, usually three, equally spaced apart around the periphery of the cutting head. The outer edges of the cutting tips provide bearing surfaces for the rotating cutter head in the member which is being bored.
The pull boring heads described cannot be operated at high speed or under heavy cutting conditions essentially because of the very limited bearing surface provided by the cutter tips. Use results in wear of the cutter tips further resulting in loss of gauge of the machined bore and still further the cutting tips tend to chatter. This in turn results in a poor quality finish to the bore, and further to a short cutting tip lifetime.
It is the object of this invention to provide a cutter head for pull boring operations which is more satisfactory than those referred to above.